Field of the Disclosure
The present disclosure relates to a technology for actively cancelling an ambient noise according to a situation in an audio output device such as a headphone.
Description of the Related Art
An audio output device such as a headphone may be equipped with various noise cancellation technologies. For example, such a headphone may collect an ambient noise via a microphone connected to a noise cancellation circuit, and may output an anti-noise (i.e. noise-canceling) signal having an opposite phase to that of the obtained noise. A mixture of the ambient noise and the anti-noise signal with the opposite phase of the noise is heard by a user, giving a resultant effect of cancelling the noise.
In the case where an audio output device supports active noise cancellation (ANC), a noise can be actively cancelled by collecting ambient noise through a microphone for ANC and determining an ambient noise environment. The audio output device may be designed such that an output unit (e.g., a speaker) cancels an ambient noise so that an audio signal provided from a playback device is clearly provided to a user.
Although application of the ANC to an audio output device brings about a noise reduction effect, an excessive application of the ANC may cause various side effects. For example, if noise reduction is maximized by applying the ANC, a distortion of a frequency response may occur, and may be noticeable to the user when the distortion exceeds a threshold. In other words, in the case where the ANC is applied to a certain degree or more, an output audio signal may be so damaged that a user may recognize it. For another example, if the ANC is enhanced, an arbitrary audio artifact may be induced by the ANC. This artifact may be, for example, a hissing noise heard by a user.
Meanwhile, a user of a headphone may desire to stay in a quiet state by wearing the headphone. For example, in a noisy place such as a bus, a subway, an airplane, or the vicinity of a construction site, the user may desire to avoid a noise by wearing the headphone to shield their ears from the noise. In some cases, the user may wear the headphone when not connected to a playback device, or, in other cases, the user may wear the headphone that is connected to the playback device, but the playback device is not currently playing any audio or video file.
However, a noise reduction technology for an audio device is mainly used for the purpose of providing an audio signal from a playback device with as little additional noise as feasible. That is, if a user does not play any audio signal or even plays an audio signal with a very weak volume while wearing a headphone to avoid an ambient noise, a conventional audio output device provides the ANC, the level of which is suitable only for a normal audio output signal (or the ANC function may not be performed at all), and thus the appropriate level of noise cancellation desired by the user is not achieved.